1. Field of the Invention
The present invention relates to a semiconductor laser, and more particularly to a semiconductor laser having an active layer with an asymmetric quantum well structure capable of improving an optical gain for a drive current-photo conversion.
2. Description of the Prior Art
Referring to FIGS. 1a to 1c, a semiconductor laser is illustrated having a double hereto (DH) structure.
As shown in FIG. 1a, the semiconductor laser comprises an n-type GaAs substrate 1 and an n-type clad layer 2 formed over the n-type substrate 1 to have a thickness of 1.0 .mu.m. The n-type clad layer 2 is made of Al.sub.x Ga.sub.1-x As. Over the n-type clad layer 2 is formed an active layer 3 which has a thickness of 250 .ANG. or less and is not doped with an impurity ion. The undoped active layer 3 is made of Al.sub.y Ga.sub.1-y As. Over the active layer 3, a p-type clad layer 4 having a thickness of 1.0 .mu.m is formed which is made of Al.sub.x Ga.sub.1-y As, similar to the n-type clad layer 2. The semiconductor laser also comprises a p-type cap layer 5 formed over the p-type clad layer 4 and made of GaAs.
In FIG. 1a, the reference, character G.sup.- denotes a p-type electrode formed at the lower surface of the substrate. 1 and the reference character G.sup.+ denotes an n-type electrode formed at the upper surface of the p-type cap layer 5.
In FIG. 1c, the reference character W1 denotes the thickness of the active layer 3. In the illustrated structure, the thickness W1 of the active layer 3 is not more than about 250 .ANG.. With this thickness W1 of active layer 3, the semiconductor laser has improvements in optical gain characteristic and thermal characteristic.
The semiconductor laser with the above-mentioned DH structure exhibits an optical gain characteristic and a thermal characteristic which are two or three times the semiconductor lasers having a general type DH structure in which an active layer has a thickness of about 1,000 .ANG..
FIG. 1b is a partial sectional view of the semiconductor laser shown in FIG. 1a. FIG. 1c is a diagram showing energy bands and quantum wells in the structure shown in FIG. 1b. As above-mentioned, the undoped active layer 3 made of Al.sub.y Ga.sub.1-y As a the thickness of not more than 250 .ANG., so that aluminum is uniformly distributed in the active layer 3. As a result, symmetric quantum wells are formed with reference to the center of the active layer 3.
The content of aluminum in the n-type clad layer 2 made of Al.sub.x Ga.sub.1-x As, the p-type clad layer 4 made of Al.sub.x Ga.sub.1-x As, and the undoped active layer 3 made of Al.sub.y Ga.sub.1-y As interposed therebetween is determined by the following equation: EQU x=y+0.3 (1)
As shown in FIG. 1c, the band gap Ex between the conduction band Ec and the valence electron band Ev, that is, an energy gap between the n-type clad layer 2 and the p-type clad layer 4 is larger than the band gap Ey of the active layer 3, since the clad layers 2 and 4 have an aluminum content larger than that of the active layer 3. As a result, the semiconductor layer has a symmetric type quantum well structure.
In conventional semiconductor lasers with such a symmetric quantum structure, the band gap Ey of the active layer 3 having a thickness of 60 .ANG. to 150 .ANG. and the band gap Ex between the n-type clad layer 2 and the p-type clad layer 4 are about 0.2 eV to about 0.3 eV.
Positive and negative voltages are applied to the p-type electrode G.sup.+ and the n-type electrode G.sup.- of the conventional semiconductor laser with the above-mentioned symmetric quantum well structure, respectively, to supply a current to the semiconductor laser. Electrons and holes are present in limited numbers in regions S1 and S1' predetermined in two quantum wells as shown in FIG. 1c, respectively. As the electrons and holes in the regions S1 and S1' are recombined, laser beams are emitted.
As mentioned above, the semiconductor laser with the quantum well structure has an advantage of increasing the electron-hole coupling effect, in that the electrons and holes are present in limited numbers in the quantum wells effectively. Accordingly, the optical gain characteristic for a drive current-photo conversion is improved.
However, the conventional semiconductor laser with the symmetric quantum well structure is subjected to a strict quantum mechanical selection condition, due to the intrinsic symmetry of its structure. That is, the transition of electrons and holes contributing to the optical gain is possible under the condition that the quantum wells are filled to a predetermined level with the electrons and holes, respectively.
Such a strict quantum mechanical selection condition determines the upper limit of optical gain obtained in the conventional semiconductor laser with the quantum well structure. As a result, a limitation exists in obtaining a high optical gain. Due to a low optical gain, it is difficult for the semiconductor laser to improve in reliance, for example, a high low-current operation characteristic and a good thermal characteristic.